Multiple memory address spaced are known in the art and include virtual memory address spaces as exemplified in U.S. Pat. No. 4,355,355 as well as real address spaced. In one example of multiple real address spaces, a first address space is used for programs and the second address space is used for data. All instruction fetches are directed toward the first address space and read/write operations are directed toward the second address space. In this way total memory accessible by a computer has been doubled in the prior art but none of the memory is protected from being read.
Another background patent of interest to this invention is U.S. Pat. No. 4,523,271 "Software Protection Method and Apparatus". In this patent, the address space is divided into a normal address space and read only memory address space. The read only memory address may, for example, be an application cartridge which is plugged into the computer. In order to prevent unauthorized copying of the application in the read only memory cartridge, circuits are added to the cartridge to prevent any read operation of the memory of the cartridge by an instruction that was not itself read from the cartridge. This circuitry allows programs in the cartridge to function normally and execute the application while at the same time preventing programs outside of the read only memory from dumping the content of read only memory as an unauthorized copy.